"Just to expand on Daniel Schoepe's answer, I do this in xmonad using the super key (the windows key) but I don't set anyting in xmonad.hs (other than setting modMask = mod4Mask). Using xmodmap I've set the left control as a second super key with the following in my .Xmodmap file:
remove Control = Control_L keysym Control_L = Super_L add Control = Control_L
You can also use xmodmap to swap capslock and control, set right control to be super as well, etc, but mind that the changes will only work in X, not in virtual terminals."
